olsmeister writes: In the seven years Cassini has spent orbiting Saturn, the spacecraft has sent back mountains of data that has changed our view of the ringed planet and its moons. Saturn's largest moon, Titan, has been a particular focus of attention because of its dense, complex atmosphere, its weather and its lakes and oceans.

Now it looks as if Titan is even stranger still. The evidence comes from careful observations of Titan's orbit and rotation. This indicates that Titan has an orbit similar to our Moon's: it always presents the same face towards Saturn and its axis of rotation tilts by about 0.3 degrees.

Together, these data allow astronomers to work out Titan's moment of inertia and this throws up something interesting. The numbers indicate that Titan's moment of inertia can only be explained if it is a solid body that is denser near the surface than it is at its centre.